Since heart failure is the leading cause of death in the western world, research into potential cell based therapies for repairing or replacing damaged cardiomyocytes is clearly important. One of the potential therapies currently being investigated is to use human embryonic stem cell derived cardiomyocyte transplantation to replace cells damaged after myocardial infarction. Here we propose experiments that will allow us to quantify and select ventricular cardiomyocytes from undifferentiated human embryonic stem cells (NIH Registry Number ES02). We will also screen cDNA libraries from pre-cardiac mesoderm and END2 cells for genes encoding cardiomyocyte differentiation inducing activity and use microarray analysis to identify novel cardiomyocyte inducing factors. These strategies will enable us to develop new methods to increase the efficiency and yield of cardiomyocyte differentiation from human embryonic stem cells and thereby obtain enough ventricular cardiomyocytes to use for transplantation into mouse models of heart disease and eventually human patients.